APPARATUS FOR AUTOMATICALLY ADJUSTING OPTICAL-AXES DIRECTION OF HEADLIGHTS OF A VEHICLE 5 Technical Field The invention relates to an apparatus for automatically adjusting optical-axes direction of headlights of a vehicle. Background Art 10 A reference herein to a patent document or other matter which is given as prior art is not to be taken as an admission or a suggestion that that document or matter was, known or that the information it contains was part of the common general knowledge as at the priority date of any of the claims. Throughout the description and claims of the specification, the word 15 "comprise" and variations of the word, such as "comprising" and "comprises", is not intended to exclude other additives, components, integers or steps. When a vehicle is laden with cargo or the like, the vehicle changes its attitude. Some vehicles are equipped with automatic headlight leveling control apparatuses to automatically adjust the optical axes of headlights in the up-and-down 20 direction in response to the changes in the vehicles' attitudes. An example of the automatic headlight leveling control apparatuses adjusts the optical-axis of each of its headlights in the following way. Firstly, vehicle-height sensors are provided to detect: the distance between the vehicle-body frame and each of the front and rear axles of the vehicle. Secondly, the pitch angle of the vehicle is calculated using: the 25 vehicle heights detected respectively by the front and the rear vehicle-height sensors; and a wheel base (i.e., the distance between the front and the rear axles). After that, the calculated pitch angle of the vehicle is compared with a reference pitch angle that has been stored in advance, and then optical-axis driving units that are incorporated respectively in the headlight units are driven to adjust the optical axes. Another 30 example of the automatic headlight leveling control apparatuses is equipped with no front-side vehicle-height sensor, and thus adjusts the optical axes in a simple manner using only the information on the vehicle-height displacement given by the rear-side vehicle-height sensor. In each of the above-mentioned cases, that is, in the case where front-side rear-side vehicle-height sensors are provided, and in the case where only a rear-side 5 vehicle-height sensor is provided, sensor output-signal characteristics that have been stored in the headlight leveling control apparatus are used to convert the sensor output signal (representing the change in voltage) outputted by each vehicle-height sensor to the amount of vehicle-height displacement. In practice, the loads with which a vehicle is laden cause deflection of the 10 vehicle-body frame, and the optical axes of the headlights change depending on the deflection amount at the front end of the vehicle-body frame. In a truck, which has a longer wheel base and which carries heavier loads, the deflection amount at the front end of its vehicle-body frame has an especially large influence on the optical axes of its headlights. 15 The above-mentioned conventional automatic headlight leveling control apparatuses, however, adjust the optical axes of the headlights without taking the deflection of the vehicle-body frames into account. Accordingly, the conventional automatic headlight leveling control apparatuses sometimes fail to achieve proper adjustment results due to the weight of the loads and/or the position where the loads 20 are placed. In addition, it is difficult to detect directly the deflection of the vehicle-body frame while the vehicle is being used. Accordingly, it is difficult for an apparatus with a simple configuration to correct the optical axis that is incorrectly directed by the deflection of the vehicle-body frame. 25 Therefore, in light of the above, it would be desirable to provide an automatic headlight leveling control apparatus which has a simple configuration and which is capable of correcting the optical-axis controlling amount by having the deflection of the vehicle-body frame reflected therein. 30 Summary of the Invention The invention provides an apparatus for automatically adjusting optical-axes *-i 2 direction of headlights of a vehicle, the apparatus including vehicle-height displacement information detecting means, storage means, inclination-angle computing means and optical-axis-direction adjusting means, the vehicle-height displacement information detecting means detecting vehicle-height displacement 5 amount of the vehicle, the storage means storing, beforehand, a predetermined arithmetic expression representing a relationship between vehicle-height displacement amount of the vehicle and an inclination angle, relative to a horizontal plane, of an optical-axis direction of a headlight of the vehicle, the inclination-angle computing means calculating the inclination angle by use of the detected vehicle 10 height displacement amount and the predetermined arithmetic expression stored in the storage means, and the optical-axis-direction adjusting means adjusting the optical-axis direction of the headlight on the basis of the inclination angle calculated by the inclination-angle computing means, wherein the storage means stores, beforehand, a predetermined correspondence relationship between the vehicle-height 15 displacement amount detected by the vehicle-height-displacement information detecting means and corrected vehicle-height displacement amount having deflection of a vehicle-body frame of the vehicle reflected therein and the inclination-angle computing means corrects the detected vehicle-height displacement amount by use of the predetermined correspondence relationship stored in the storage means, and 20 calculates the inclination angle assigning the corrected vehicle-height displacement amount to the predetermined arithmetic expression stored in the storage means. The vehicle-height-displacement information detecting means may be a vehicle-height sensor that outputs a sensor output signal. An output value of the vehicle-height sensor charges according to the vehicle-height displacement amount. 25 In addition, the predetermined arithmetic expression stored in the storage means represents the relationship between vehicle-height displacement amount of the vehicle without a reflection of the vehicle-body frame and the inclination angle, the predetermined correspondence relationship stored in the storage means is a corrected sensor output-signal characteristics which have been obtained by multiplying a 30 sensor output-signal characteristics by a correction value, the sensor output-signal characteristics is approved between the output value of the vehicle-height sensor and 3 the vehicle-height displacement amount, and the correction value is set by having the deflection of the vehicle-body frame reflected therein. With the above-described configuration, the inclination angle of the headlight is calculated using: the predetermined arithmetic expression representing the 5 relationship between the vehicle-height displacement of the vehicle and the inclination angle of the optical-axis direction of the headlight of the vehicle; and the predetermined correspondence relationship (e.g., sensor output-signal characteristics) set so as to have the deflection of the vehicle-body frame reflected therein. Then, the optical-axis direction of the headlight is adjusted on the basis of the calculated 10 inclination angle. In addition, a publicly-known arithmetic expression may be used as the predetermined arithmetic expression representing the relationship between the vehicle-height displacement information of the vehicle and the inclination angle of the optical-axis direction of the headlight of the vehicle. To put it differently, the use of the existing arithmetic expression and the pre-set, predetermined correspondence 15 relationship (e.g., sensor output-signal characteristics that have been corrected by use of the pre-set correction value) allows a more accurate optical-axis control in which the deflection of the vehicle-body frame is reflected, to be executed without relying on any complex processing or a complex configuration of the apparatus. In addition, the sensor output-signal characteristics stored in the storage 20 means may be a correction value capable of making the inclination angle calculated by the inclination-angle computing means fall within the predetermined range for all of the plurality of hypothetical loading patterns that differ from one another in the way how load is carried by the vehicle-body frame. With the above-described configuration, although the loading conditions of 25 the load (the position where the load is placed and/or the weight of the load) may vary from one occasion to another even for the same vehicle, the optical-axis correcting control to be executed is made capable of directing the optical axis of the headlight within a predetermined range in all the occasions. In addition, the sensor output-signal characteristics stored in the storage 30 means may be a correction value set so as to have tire deflection and suspension deflection reflected therein. 4 The above-described configuration allows, without any additional processing that has to be executed by the automatic headlight leveling control apparatus, execution of the optical-axis correction control in which not only the deflection of the vehicle-body frame but also the tire deflection and the suspension deflection are 5 reflected. Effects of the Invention According to the invention, the optical-axis correction in which the deflection, of the vehicle-body frame is reflected can be executed with a simple 10 configuration. Brief Description of the Drawings [Fig. 1] Fig. I is a schematic diagram illustrating the configuration of a vehicle equipped with an automatic headlight leveling control apparatus according to 15 a first embodiment of the invention. [Fig. 2] Fig. 2 is a block diagram illustrating the configuration of the automatic headlight leveling control apparatus illustrated in Fig. 1. [Fig. 3] Fig. 3 is a flowchart illustrating the processes executed by an ECU of the automatic headlight leveling control apparatus illustrated in Fig. 1. 20 Description of Symbols I vehicle 2 front-side vehicle-height sensor (vehicle-height-displacement information detecting means) 3 rear-side vehicle-height sensor (vehicle-height-displacement information 25 detecting means) 4 ECU 5 headlight units (headlights) 9 optical-axis driving unit (optical-axis-direction adjusting means) 11 computing section (inclination-angle computing means) 30 12 storage section (storage means) <me"""" 5 Best Modes for Carrying Out the Invention An embodiment of the invention will be described below by referring to the drawings. Fig. I is a schematic diagram illustrating the configuration of a vehicle 5 equipped with an automatic headlight leveling control apparatus according to a first embodiment of the invention. Fig. 2 is a block diagram illustrating the configuration of the automatic headlight leveling control apparatus illustrated in Fig. 1. Fig. I shows that a vehicle 1 equipped with the automatic headlight leveling control apparatus of this embodiment is a cab-over-engine type truck. Headlight units 10 (headlights) 5 are provided in the front-end portion of the vehicle 1. The automatic headlight leveling control apparatus includes a front-side vehicle-height sensor 2, a rear-side vehicle-height sensor 3, and an electronic control unit (ECU) 4. The optical-axis direction of each headlight 5 is affected by the inclination of a vehicle-body frame 7. The optical-axis direction of each headlight unit 5 is affected 15 also by the deflection of the vehicle-body frame 7 (simply referred to as "frame deflection") caused by the loads with which the vehicle 1 is laden. In addition, the optical-axis direction of each headlight 5 is affected by various other factors than the frame deflection--for example, the deflection of the tires. A generic term "frame deflection and the like factors" used in the following descriptions means both the 20 frame deflection and the above-mentioned various other error factors than frame deflection. Suppose, for example, that the optical-axis direction in a state where the vehicle-body frame 7 is deflected by a load 15 as shown in Fig. I has to be controlled so as to be parallel with the corresponding optical axis in a state where the vehicle 1 is laden with no load. The control amount (angle 01) in a case where the control is 25 executed by considering the inclination of the vehicle-body frame 7 as well as the frame deflection and the like 6 419000060AU_specificationNonamendment factors differs from the control amount (angle 0) in a case where the control is executed by considering (detecting) only the inclination of the vehicle-body frame, that is, without considering the frame deflection (specifically, 0 < 01). The ECU 4 of this embodiment executes optical-axis control-amount computing processing by considering not only the inclination of the vehicle-body frame 7 but also the deflection of the vehicle-body frame 7 and the like factors in order to reduce the influence that the deflection and the like factors have on the optical-axis direction control. The front-side vehicle-height sensor 2 detects the displacement amount of the distance between a front axle 6 and the vehicle-body frame 7 (simply referred to as the "front-side vehicle-height displacement amount"), and outputs a front-side-sensor output signal (representing the change in electric voltage) corresponding to the front-side vehicle-height displacement amount to the ECU 4. Likewise, the rear-side vehicle-height sensor 3 detects the displacement amount of the distance between a rear axle 8 and the vehicle-body frame 7 (simply referred to as the "rear-side vehicle-height displacement amount"), and outputs a rear-side-sensor output signal (representing the change in electric voltage) corresponding to the rear-side vehicle-height displacement amount to the ECU 4. Each of the headlight units 5 includes: a lamp (not illustrated); a reflector (not illustrated) to which the lamp is fixed; and an optical-axis driving unit (actuator) 9. The reflector is swingably supported by the optical-axis driving unit 9, which includes, among other things, a stepping motor to change the inclination of the reflector and thus to adjust the optical-axis direction of the lamp (the optical-axis direction of the headlight unit 5). As Fig. 2 shows, the ECU 4 includes a computing section 11, a storage section 12, and a control-signal output section 13. The functions of the computing section 11 and the control-signal output section 13 are implemented by, for example, a central processing unit (CPU). The function of the storage section 12 is implemented by a storage medium such as a read only memory (ROM) or a random access memory (RAM). The storage section 12 stores, in advance, a program of the processing for 7 419000060AU_specificationNonamendment calculating the optical-axis control amount (arithmetic-operation program) to be executed by the computing section 11. In addition, the storage section 12, in advance, stores: the front-side-sensor output-signal characteristics; the rear-side-sensor output-signal characteristics; the front-side-sensor voltage value for unladen vehicle; the rear-side sensor voltage value for unladen vehicle; the wheel base WB; and information on the optical-axis driving unit 9 (optical-axis driving-unit information). The program of the processing for calculating the optical-axis control amount includes various arithmetic expressions to be used when the optical-axis control amount is calculated on the basis of the front-side vehicle-height displacement amount and the rear-side vehicle-height displacement amount. Note that the arithmetic expressions are not defined by having the deflection of the vehicle-body frame 7 and the like factors caused by the loads reflected therein. Detailed descriptions will be given later as to the deflection of the vehicle-body frame 7 and the like factors caused by the loads. The front-side-sensor output-signal characteristics include an arithmetic expression, a correlation map, or the like representing the relationship between the front-side vehicle-height displacement amount and the value of the sensor output signal, of the front-side vehicle-height sensor 2. The rear-side-sensor output-signal characteristics include an arithmetic expression, a correlation map, or the like representing the relationship between the rear-side vehicle-height displacement amount and the value of the sensor output signal, of the rear-side vehicle-height sensor 3. The characteristic values of the sensor output signals of the front-side vehicle-height sensor 2 and of the rear-side vehicle-height sensor 3 are superposed (multiplied) by their respective coefficients (%) to obtain their respective optical-axis control amounts in which the deflection of the vehicle-body frame 7 and the like factors (frame deflection and the like factors) caused by the loads are taken into consideration. Here, the coefficients are a correction value for the front-side-sensor output-signal characteristics and a correction value for the rear-side-sensor output-signal characteristics. Even for the same vehicle 1, the frame deflection and the like factors differ from one occasion to another depending upon the weight of the loads 15, the positions 8 419000060AU_specificationNonamendment where the loads 15 are placed, and the like (these variables will be simply referred to as the "loading pattern"). The inclination in the optical-axis direction, which derives from the frame deflections and the like factors, varies from one occasion to another depending upon the loading patterns. For this reason, a simulation is carried out assuming various hypothetical loading patterns. Specifically, the frame deflection and the like factors under each of the hypothetical loading patterns are obtained by an experiment and/or a mathematical operation. Then, the influence of the frame deflection and the like factors under each hypothetical loading pattern is converted to a combination of a hypothetical front-side vehicle-height displacement amount and a hypothetical rear-side vehicle-height displacement amount. Subsequently, to provide each of the hypothetical vehicle-height displacement amounts to the computing section 11, the coefficients to be superposed respectively on the front-side-sensor output-signal characteristics and on the rear-side-sensor output-signal characteristics are obtained. Note that the coefficients thus obtained have to be values that are applicable commonly to all the occasions under the various different hypothetical loading patterns. Whether the values thus obtained are applicable or not is determined in the following way. Firstly, the optical-axis control amount is calculated using the values obtained in the above-described way, the front-side sensor output signal from the front-side vehicle-height sensor 2 and the rear-side sensor output signal from the rear-side vehicle-height sensor 3. Then, a determination is executed as to whether the optical-axis control amount thus calculated is or is not within a predetermined range. The front-side-sensor voltage value for unladen vehicle is the value of the front-side-sensor output voltage value while the vehicle I is mounted with no loads on the platform (in the unladen state). The rear-side sensor voltage value for unladen vehicle is the value of the rear-side-sensor output voltage in the unladen state. These front-side-sensor voltage value for unladen vehicle and rear-side-sensor voltage value for unladen vehicle are stored for each individual vehicle by carrying out initializing operation (initial learning operation) of the ECU 4 under predetermined conditions while the vehicle is in the unladen state. The wheel base WB is the distance between the front axle 6 and the rear axle 8. The optical-axis driving-unit information is a 9 control-amount calculation parameter related to the structure of the optical-axis driving unit (actuator) 9 incorporated in each headlight unit 5. The computing section 11, theoretically, calculates the control amount in the following way. Firstly, the inclination of the vehicle-body frame 7 (referred to as the 5 "vehicle-attitude change amount") is calculated using: the front-side vehicle-height displacement amount based on the front-side-sensor output signal transmitted from the front-side vehicle-height sensor 2; and the rear-side vehicle-height displacement amount based on the rear-side-sensor output signal transmitted from the rear-side vehicle-height sensor 3. Then, the control amount needed for restoring the original 10 optical direction that has been altered by the influence of the inclination is calculated using the vehicle-attitude change amount thus calculated. In practice, however, such error factors as the frame deflection and the like factors are influential to a certain, non-negligible extent. Accordingly, the proper optical-axis control amount that is actually needed differs from the optical-axis control amount based on the vehicle 15 attitude change amount calculated from the front-side and the rear-side vehicle-height displacement amounts. The control-signal output section 13 generates a control signal for driving the optical axis corresponding to the optical-axis control amount calculated by the computing section 11. The calculated control signal for driving the optical axis is 20 outputted to the optical-axis driving unit 9. The optical-axis driving unit 9 adjusts the optical-axis direction according to the control signal for driving the optical axis thus received. Subsequently, the optical-axis control-amount computing processing executed by the computing section 11 will be described by referring to the flowchart illustrated 25 in Fig. 3. This processing is repeatedly executed every predetermined length of time while the ignition key is in the ON state. The processing starts with the reception of the front-side sensor output signal from the front-side vehicle-height sensor 2 and the rear-side sensor output signal from the rear-side vehicle-height sensor 3 (step SI). 30 Next, various pieces of information are retrieved from the storage section 12 <M"C 10 419000060AU_specificationNonamendment (step S2). These retrieved pieces of information include: the front-side-sensor output-signal characteristics; the rear-side-sensor output-signal characteristics; the front-side-sensor voltage value for unladen vehicle; the rear-side sensor voltage value for unladen vehicle; the wheel base WB; and the optical-axis driving-unit information. The stored front-side-sensor output-signal characteristics and the stored rear-side-sensor output-signal characteristics are respectively the ones defined by considering the hypothetical vehicle-height displacement amount obtained by converting, quantitatively, the influence of such error factors as the flame deflections and the like factors. What follows next is the acquisition of the front-side vehicle-height displacement amount for unladen vehicle AHf and the rear-side vehicle-height displacement amount for unladen vehicle AHr (step S3). Specifically, the front-side-sensor output signal received at step S1 is checked against the front-side-sensor output-signal characteristics acquired at step S2, and thus the corresponding front-side vehicle-height displacement amount AHf is acquired. Likewise, the rear-side-sensor output signal received at step S1 is checked against the rear-side-sensor output-signal characteristics acquired at step S2, and thus the corresponding rear-side vehicle-height displacement amount AHr is acquired. The acquired front-side vehicle-height displacement amount AHf and the acquired rear-side vehicle-height displacement amount AHr include values in which the influence of such error factors as the frame deflection and the like factors are reflected quantitatively by being converted, hypothetically, to the front-side and the rear-side vehicle-height displacements. What follows next is the calculation of the vehicle-attitude change AO (step S4). Specifically, the value of the front-side vehicle-height displacement amount AHf and the value of the rear-side vehicle-height displacement amount AHr, both of which have been acquired at step S3, and the stored value of the wheel base WB are assigned to their respective variables in the following Equation (1). AO [deg]= tan"' {(AHr - AHf) / WB}... (1) I1 419000060AUspecificationNonamendment As the Equation (1) reveals, the vehicle-attitude change AO is obtained using quite a simple trigonometric formula defined on the assumption that the vehicle has ideal stiffness without any such error factors as the frame deflection and the like factors. Remember that the front-side vehicle-height displacement amount AHf and the rear-side vehicle-height displacement amount AHr are the values in which the influence of such error factors as the frame deflection and the like factors are reflected by being converted, hypothetically, to the front-side and the rear-side vehicle-height change amounts. Accordingly, the value of the vehicle-attitude change AO acquired in this way contains the influence of these error factors. Finally, the optical-axis control amount 0 is calculated (step S5). Specifically, the optical-axis control amount 0 is determined so as to cancel out (nullify) the optical-axis change amount caused by the vehicle-attitude change AO calculated at step S4. As has been described thus far, according to this embodiment, the optical-axis control amount 0 is calculated in the following way. Firstly, a correction is executed. Specifically, the front-side-sensor output-signal characteristics of the front-side vehicle-height sensor 2 (the original output-signal characteristics of the sensor in which the frame deflection and the like factors are not reflected) and the rear-side-sensor output-signal characteristics of the rear-side vehicle-height sensor 3 (the original output-signal characteristics of the sensor in which the frame deflection and the like factors are not reflected) are multiplied by their respective correction values so that the deflection of the vehicle-body frame 7 and the like factors can be reflected in the values of the above-mentioned characteristics. Secondly, the front-side vehicle-height displacement amount and the rear-side vehicle-height displacement amount are calculated using the corrected front-side-sensor output-signal characteristics and the corrected rear-side-sensor output-signal characteristics, respectively. Thirdly, the front-side vehicle-height displacement amount for unladen vehicle AHf and the rear-side vehicle-height displacement amount for unladen vehicle AHr are calculated using the acquired front-side vehicle-height displacement amount and the acquired rear-side vehicle-height displacement amount, respectively. Fourthly, the vehicle-attitude 12 change AO is calculated using the calculated front-side vehicle-height displacement amount AHf, the calculated rear-side vehicle-height displacement amount AHr, and the Equation (1). Then, the optical-axis control amount 0 is calculated so as to cancel 5 out the optical-axis change amount caused by the vehicle-attitude change AO. This Equation (1) has been publicly known as an arithmetic expression representing the relationship between the vehicle-height displacement amount of the vehicle I and the inclination-angle displacement amount for the headlight optical-axis direction of the vehicle 1. In summary, this embodiment employs the existing arithmetic expression, 10 and the corrected front-side-sensor output-signal characteristics and the corrected rear-side-sensor output-signal characteristics. These corrected sensor output-signal characteristics have been set and stored, after being corrected using their respective accurate correction values that had been calculated, for example, through a simulation. For this reason, this embodiment allows a more accurate optical-axis control in which 15 the deflection of the vehicle-body frame 7 and the like factors are reflected, to be executed without relying on any complex processing or a complex configuration of the apparatus. In addition, for all of the plural hypothetical loading patterns, in which the conditions how the vehicle-body frame 7 carries the load differ from one pattern to 20 another, the correction values for the front-side sensor output-signal characteristics and for the rear-side sensor output-signal characteristics are set so that the optical-axis direction after the control using the calculated optical-axis control amount can be within a predetermined range. Accordingly, although the loading conditions of the load 15 (the position where the load 15 is placed and/or the weight of the load 15) may vary 25 from one occasion to another even for the same vehicle 1, the optical axis of each headlight 5 can be directed within a predetermined range in all the occasions. Moreover, the influence of the tire deflections and the like factors are reflected in the correction values for the front-side-sensor output-signal characteristics and for the rear-side-sensor output-signal characteristics. Accordingly, the influence of plural 30 error factors such as the deflection of the vehicle-body frame 7 and the like factors can be taken into consideration when the optical-axis control is executed. 13 Next, a second embodiment of the invention will be described. The apparatus according to the second embodiment has no front-side vehicle height sensor 2 though the apparatus according to the first embodiment has one. In the second embodiment, a computing section 11 calculates the optical-axis control 5 amount on the basis of the rear-side vehicle-height displacement amount alone detected by a rear-side vehicle-height sensor 3. For this reason, the components that have their respective counterparts in the apparatus of the first embodiment are denoted by the same reference numerals used in the first embodiment, and no detailed description for those components will be repeated in this second embodiment. 10 The automatic headlight leveling control apparatus of this embodiment includes the rear-side vehicle-height sensor 3 and an electronic control unit (ECU) 4. The ECU 4 includes the computing section 11, a storage section 12, and a control signal output section 13. The storage section 12 stores, in advance, a program of the processing for calculating the optical-axis control amount. In addition, the storage 15 section 12, in advance, stores: the rear-side-sensor output-signal characteristics; the rear-side sensor voltage value for unladen vehicle; the wheel base WB; and the optical axis driving-unit information. The program of the processing for calculating the optical-axis control amount includes various arithmetic expressions to be used when the optical-axis control amount is calculated on the basis of the rear-side vehicle-height 20 displacement amount. Note that, as in the case of the first embodiment, the arithmetic expressions are not defined by having the frame deflection frame 7 and the like factors reflected therein. The computing section 11 calculates the optical-axis control amount using the rear-side vehicle-height displacement amount based on the rear-side-sensor output 25 signal transmitted from the rear-side vehicle-height sensor 3. Specifically, firstly, the rear-side vehicle-height displacement amount for unladen vehicle AHr is acquired in a similar manner to that in the first embodiment. Then, the vehicle-attitude change AO is calculated by assigning the acquired value of rear-side vehicle-height displacement amount AHr and the stored value of the wheel base WB to their respective variables in 30 the following Equation (2). The optical-axis control amount 0 is then determined so as 14 419000060AUspecificationNonamendment to cancel out (nullify) the optical-axis change amount caused by the vehicle-attitude change AO calculated above. In this case, the front-side vehicle-height displacement amount has a certain influence on the actual vehicle attitude, and the influence is, inevitably, a source of error for the optical-axis control. The optical-axis correction control can be executed using the correction value containing not only the frame deflection and the like factors but also the error caused in the above-mentioned way. AO [deg] = tan-' (AHr / WB)... (2) What will be described next is an exemplar method of acquiring, through a simulation, the correction value for the rear-side-sensor output-signal characteristics in this embodiment. As described above, even for the same vehicle 1, the frame deflection and the like factors vary from one occasion to another depending upon the loading pattern such as the weight of a load 15 and the position where the load 15 is actually placed. Accordingly, the inclination in the optical-axis direction, which derives from the frame deflections and the like factors, varies from one occasion to another depending upon the loading patterns. For this reason, various hypothetical loading patterns are firstly assumed so as to differ from one another in the position where the load is placed and in the size of the load. For example, in each of the loading pattern a first predetermined load is placed at the front-end position of the platform; a second predetermined load is placed at a position located at the rear of the front end by an amount equivalent to a quarter of the platform length; a third predetermined load is placed at the center position, in the front-to-rear direction, of the platform; and a fourth predetermined load is placed at a position right above the rear axle. The preferable setting of the position of the load includes both the cases of the concentrated load and the distributed load whereas the preferable setting of the size (weight) of the load includes a case where the total weight of the loads has a value close to the maximum loading capacity for the vehicle 1. Once plural hypothetical loading patterns have been assumed, a light emitting angle after the optical-axis control is calculated using the specifications of the vehicle I 15 and the following Equations (3) to (5). Here, the optical-axis control is executed using the correction value of the rear-side sensor output-signal characteristics as the parameter. Note that the value of the emitting angle and that of the optical-axis control amount in Equation (3) are their respective inclinations (%). 5 Emitting angle after optical-axis control = emitting angle before optical-axis control + necessary optical-axis control amount calculated by ECU ... (3) Emitting angle before optical-axis control = initial adjustment angle for 10 unladen vehicle + optical-axis emitting-angle change amount influenced by carrying loads ... (4) Necessary optical-axis control amount calculated by ECU = - ((displacement of the rear-suspension spring directly detected by rear-side vehicle-height sensor / 15 wheel base WB) x correction value for rear-side-sensor output-signal characteristics} ... (5) In these equations, the initial adjustment angle for unladen vehicle is a set value of the initial optical-axis emitting angle adjusted mechanically by use of an optical-axis 20 tester while the vehicle is in the unladen state. The value of the initial adjustment angle for unladen vehicle is set beforehand for each vehicle. The optical-axis emitting-angle change amount influenced by carrying loads is the amount of change in inclination of the optical-axis emitting angle from a time when the vehicle is in the unladen state to a time when the vehicle is laden with some loads. The change in the optical-axis 25 emitting-angle inclination is caused by the deflection of the vehicle-body frame, the deflection of the tires, and the displacements of the suspension springs. The optical axis emitting-angle change caused by the deflection of the vehicle-body frame takes place when the warpage of the vehicle-body frame changes the inclination of the front-end portion of the vehicle-body frame (the positions where the headlights are 30 fitted). The deflections and the displacements mentioned above are calculated, for example, using: the assumed hypothetical loading pattern (the load-input position and M-C>16 the size of the load); the shape and the material of the vehicle-body frame; the spring constant of each tire; and the spring constant of each suspension. Once the emitting angle after optical-axis control with the correction value for 5 the rear-side-sensor output-signal characteristics being used as the parameter has been calculated for each hypothetical loading pattern, the correction value (%) for the rear side-sensor output-signal characteristics has only to be acquired so that the emitting angle after optical-axis control can be within a desired range for all the hypothetical loading patterns. 10 Note that the above-described embodiments are examples of the invention. Accordingly, the invention is never limited by the above-described embodiments. Various modifications that are not described in the above-described embodiments can be made as long as such modifications do not depart the technical ideas of the invention. 15 Industrial Applicability The invention is applicable to an automatic headlight leveling control apparatus to be mounted on a vehicle. 17